A mixed cellular and D2D system allows devices to directly exchange information with each other, i.e. without passing through a base station and a core network as is common with conventional cellular communication systems. An exemplifying mixed cellular and D2D system, e.g. featuring D2D Proximity Services (D2D ProSe), was introduced with Release-12 (Rel-12) of Long Term Evolution (LTE) of the Third Generation Partnership Project (3GPP). With D2D ProSe, two main services for D2D communication are provided; a direct communication service and a direct discovery service.
With the direct communication service, the devices establish a direct user plane connection. The direct communication service primarily includes voice, such as Voice over Internet Protocol (VoIP), and file transfer, such as file transfer over File Transfer Protocol (FTP). With the direct discovery service, the devices transmit and monitor signals that enable them to be aware of proximity to another device or other devices.
Use cases for D2D ProSe include both Public Safety (PS) and commercial use.
In the PS applications, the direct communication service provides basic VoIP connectivity to other devices, independently of cellular coverage, such as LTE coverage, for the devices. Moreover, the direct discovery service may provide functions for detection of presence of peer devices, e.g. based on their professions or the like. Typically, in an emergency situation, it would be beneficial to easily find out if there is any doctor and/or nurse nearby.
In the commercial applications, the direct communication service provides simple push-to-talk functions, where communication typically is be one-to-many. Furthermore, the direct discovery service can broadcast advertisements, such as “pizza for 5$”, etc.
In a known mixed cellular and D2D system according to 3GPP Rel-12, a user transmission is sent from a sender device to a receiver device at a specific transmission power for D2D transmissions, i.e. D2D transmissions on so called D2D links between e.g. the sender device and the receiver device.
For cellular transmissions in an LTE system, it is known to set the transmission power according to the following formula:Ptx=min{Pmax,P0+10·log(M)+∝·PL+δmcs+f(Δi)  (1)
where:
Pmax: Maximum power allowed by the transmission in for uplink. It depends on the UE category,
M: The number of allocated Physical Resource Blocks (PRBs) per user,
P0: The power to be contained in one PRB. It is cell specific parameter and measured in dBm/PRB,
α: Path loss compensation factor. It is a cell specific parameter in the range [0 1],
PL: Estimated uplink path loss at the UE,
δmcs: MCS dependent offset. It is UE specific, and
f(Δi): closed loop correction function.
However, this formula does not efficiently set the transmission power for D2D transmissions. Typically, the transmission power for D2D transmissions are set to a constant value below an upper power limit, which depends on pathloss. Hence, a disadvantage is that it may be difficult to set the transmission power for D2D links such that improved D2D communication may be achieved.